The present invention relates generally to the field of flow measurement of gases. More particularly, it relates to the measurement of gas flow where accuracy is important, such as in a medical ventilator or ventilation monitor.
Many types of flow sensing systems have been used to measure the gas flow in pressurized gas systems, such as those in a ventilator. One commonly used method is called a hot wire anemometer in which a bridge circuit is used and the magnitude of the current to a hot wire in the gas flow gives an indication of the gas flow.
The current flow through the hot wire is a function of the gas velocity as well as other factors such as the transport properties of the gas (thermal conductivity, density and viscosity) and the temperatures of the gas and the hot wire. The resistance of the wire varies with the temperature of the wire for commonly used wires such as platinum.
Early versions of the hot wire anemometer measured the hot wire sensor current while trying to maintain a constant resistance in the sensor. This was not particularly accurate because the current, while a function of flow rate, is also to some extent dependent on the temperature and transport properties of the gas. Thus the output would be accurate only for one gas within a narrow temperature range.
To help improve the accuracy, this configuration has been modified to provide for a constant temperature difference (delta) between the hot wire sensor and a similar sensor substantially unaffected by flow rate which is used to sense the temperature of the gas. However, even these constant temperature difference hot wire flow sensor systems do not necessarily provide the accuracy needed for some applications, in that they are still sensitive to changes in the transport properties of the gas such as thermal conductivity, density and absolute viscosity, and are also somewhat sensitive to gas temperature changes. Accuracy in such systems is particularly compromised where the gas composition changes and/or the water vapor content is significant (i.e. humidified gas).
More recent examples of flow sensors that have somewhat effective temperature compensation arrangements are disclosed in U.S. Pat. Nos. 4,807,151 and 4,854,167. However, both of these are limited in their ability to accurately temperature compensate because they require the use of bridge circuits.